The arabesque in Poe signals an upward movement towards the eternal, the infinite, the spiritual.
The grotesque (“the strange, fantastic, ugly, incongruous, unpleasant, or disgusting”) is essentially the same thing in perhaps a different key: both are adornments for cathedrals and mosques, and so suggest this ascending movement to the unity of the eternal through combining and uniting very diverse and incongruous images.
– Dr. J. Adamson
Wikipedia entry on “Arabesque”: http://en.wikipedia.org/wiki/Arabesque_(Islamic_art)
Wikipedia entry on “Grotesque”: http://en.wikipedia.org/wiki/Grotesque